Poison damage is actually extremely similar to the Physics Impact damage type. It is almost always (currently) included in the enemy list of damage types to not apply armor to, and it has no damage type multipliers to do a flat bonus or reduction to it.

In general, this damage type is used more for themed damages rather than an actual balancing aspect. This is because the only major difference it has with Physics Impact is the inability to pin corpses to walls or harm other enemies with said corpses. To follow the theme, some weapons apply poison damage over time, although this doesn't depend the damage type itself.

Poison Mods

There is no mod that add poison elemental damage yet, but Saryn'sContagion will synthesize the 'element' onto your melee weapons upon use.

There is a mod named Antitoxin that reduces the effect of poison from sources such as the Toxic Ancient

Notes

Contrary to popular belief, MOAs (and Ambulas) do not have a specific damage multiplier for Poison Damage. Instead, their torsos (aka fanny pack) have a 3x damage multiplier for all damage types, including Poison.